The Effect of Vertical Irregularities on the Seismic Behavior of the Moment Steel Frames
The behavior of irregular buildings in the past earthquakes shows that they had an improper behavior in relation to regular buildings. In this regard, in the present research, the seismic behavior of the special moment-resisting steel frames with irregular distribution of mass in the height has been investigated. This assessment has been performed using linear and nonlinear structural analysis such as the modal linear, static, and dynamic analysis and the history nonlinear, static, and dynamic analysis and with the help of Etabs9, and Sap2000 version 14.2.2 software. In this research, the distribution parameters of the plastic joints and the relative displacement, basic cutting-roof displacement and seismic behavior of the structure which has a special importance in the calculation of the amount of the structure vulnerability have been investigated. So, 14 frames (in which parameters of the floors numbers (6, 18) and situation of the irregular floor (below and middle), weight ratio of the irregular floor to the adjacent classes (50%, 100%) have been have varied) have been investigated in the form of a two-dimensional model. In this regard, to investigate nonlinear behavior of the above structures, nonlinear behavior model of the FEMA356 and nonlinear dynamic analysis with the help of the Bam earthquake records and nonlinear static analysis with two kinds of load distribution have been used. Result of this research shows that the increase of the floors mass has led to the increase of the relative displacement of the floors and increase of the plastic joints in the irregular floor specially in the irregularity of the last floor and for this reason, the reinforcement of the beams and columns of irregular floor and its around floors must be done and the increase of the roof floor mass must be avoided.